1. Field of the Invention
This invention relates to a process for the preparation of anhydrous niobium and tantalum pentafluorides. More specifically, the invention relates to reacting a niobium or tantalum pentoxide with an excess of anhydrous hydrogen fluoride in the presence of a dehydrating agent such as to produce catalytically active anhydrous niobium or tantalum pentafluoride.
2. Description of the Related Art Including Information Disclosed Under 37 CFR .sctn..sctn.1.97-1.99
It is generally known that tantalum pentafluoride (TaF.sub.5) and niobium pentafluoride (NbF.sub.5) are useful in the petroleum processing industry as isomerization and alkylation catalysts. Tantalum pentafluoride and niobium pentafluoride are also useful as fluorination catalysts in the preparation of chlorofluorinated hydrocarbons by catalyzing the addition of hydrogen fluoride or by catalyzing the exchange reaction of fluorine for chlorine in chlorocarbons or chlorohalocarbons.
Various methods of preparing tantalum and niobium pentafluorides have been proposed. Typically they are prepared by passing fluorine gas over niobium or tantalum metals or chlorides at elevated temperatures; however, the high costs associated with the production of niobium and tantalum metals and the expensive use of elemental fluorine with the special equipment required to compensate for the corrosive and reactive nature of the gas make niobium and tantalum pentafluorides prepared by this method very expensive. It is also generally known in the prior art that the catalytic activity of tantalum and niobium pentafluoride gradually decline due to the accumulation of various contaminants or poisons and in particular because of the accumulation of water or other oxygen containing compounds. Thus, there is an incentive to regenerate a spent tantalum or niobium pentafluoride catalyst and various methods of accomplishing this have been proposed.
For example, in a pair of U.S. Pat. Nos., 4,098,833 and 4,120,912, methods of regenerating liquid phase Friedel-Crafts type hydrocarbon conversion catalyst comprising a metal halide (including tantalum and niobium fluoride) used with a Bronsted acid (such as HF) are described. In the first reference, the spent catalyst is contacted with a noble metal and hydrogen at 0.degree. to 150.degree. C. while in the second reference the spent catalyst is contacted with hydrogen at a partial pressure of at least one atmosphere and at a temperature of 100.degree. to 500.degree. C. In U.S. Pat. No. 4,469,804 the regeneration of a niobium or tantalum catalyst chemically bonded to solid support wherein the supported catalyst has been deactivated by contact with oxygen or a compound containing oxygen is disclosed. In this regeneration process the spent solid catalyst and support are contacted with a liquid or gaseous halogenated hydrocarbon at conditions that thermodynamically favor the conversion of niobium or tantalum pentoxide to niobium or tantalum pentahalide.
U.S. Pat. No. 4,124,692 discloses a method of preparing and regenerating anhydrous TaF.sub.5 from a mixture of water and fluorotantalic acids using a dehydrating agent. In this reference an aqueous mixture of tantalum oxide and tantalum oxyhalide is first reacted with hydrogen fluoride to produce a mixture of water and fluorotantalic acids. After removing excess HF, a dehydrating agent, such as phosgene or chloroform, is then added to react with the water. According to this prior art reference, the dehydration reaction is effective when the ratio of oxygen to tantalum is about 1.5 or less and when an oxygen to tantalum ratio of 2 or more is present and the temperature is about 100.degree. C. the mixture of water and fluorotantalic acids decomposes to form an oxyfluoride and HF, thus representing conditions at which the dehydration reaction will not proceed satisfactorily. In the case of a mixture of water and fluorotantalic acids formed from the reaction of tantalum pentoxide, Ta.sub.2 O.sub.5, and HF the oxygen to tantalum ratio is inherently 2.5. Therefore, the prior art reference teaches that the ratio must be adjusted downward, preferably to 1.25, by the addition of 2 moles of anhydrous TaF.sub.5 for every mole of Ta.sub.2 O.sub.5 starting material. This addition of anhydrous TaF.sub.5 must be done before the addition of a dehydrating agent, consequently this prior art process is essentially a three-step or stage process.